1. Field of the Invention
The present invention relates to a liquid level control system, and more particularly to a system for controlling the level of liquid stored in a steam drum containing a steam-liquid separator that separates liquid from two-phase fluid containing steam and liquid generated in a boiler.
2. Description of the Related Art
Power stations, such as thermal or nuclear power stations, generate steam by heating water or other liquid with a boiler, and obtain electrical power by driving a turbine and an electric generator with the generated steam. In most cases, however, the steam generated by the boiler is a two-phase flow that contains steam and water (liquid/mist).
Meanwhile, the turbine, which uses steam, needs dry steam, which is water-free, in order to maintain its soundness. Therefore, there is a known method for introducing the two-phase flow, which is generated in the boiler, into a steam drum, separating water from the two-phase flow with steam-water separators mounted in the steam drum, and forwarding the water-free steam to the turbine.
The water separated by the steam-water separators is retained in the steam drum. Further, a feed-water pump supplies water from a steam condenser to the steam drum in order to balance with the flow rate of steam flowing to the turbine. The separated water retained in the steam drum and the water fed from the steam condenser are both supplied to the boiler.
If, for instance, the output of electric power by a power station is changed or the load on the power station is suddenly decreased or lost, the flow rate of steam flowing from the steam drum to the turbine rapidly changes. The balance between a feed water flow rate and steam flow rate then becomes impaired to change the volume of water in the steam drum, thereby causing the water level to change.
However, the water level in the steam drum has to be maintained within a predetermined range for the following reasons:    (1) If the water level is raised, the steam flowing into the turbine contains a large quantity of moisture (mist) due to the characteristics of the steam-water separators. This may cause damage to the turbine.    (2) If the water level is lowered, the resulting exposed portion of the boiler is dried out. This may cause damage to the boiler.
In view of the above circumstances, a known technology disclosed, for instance, in JP-5-265569-A makes it possible to detect the level of water retained in the steam drum, the flow rate of water fed from the steam condenser to the steam drum, and the flow rate of steam fed from the steam drum to the turbine, and adjust the water level in the steam drum to a predetermined level by regulating the flow rate of water fed from the steam condenser in accordance with the above three detected signals.